A variety of devices are available on the market that are designed to protect those devices that are susceptible to damage by voltage surge when the voltage applied between their power terminals exceeds a maximum acceptable threshold.
In this sense, the devices most used are MOV, based on semiconductors and the like, as well as GDT devices. The use of hybrid arrestors based on a combination of MOV and GDT devices is also known.
MOV devices are fast acting which is very desirable in certain applications, but with the inconvenience of not being able to absorb an unlimited number of surges, that is, they degrade with use and in the end fail. The number of times an MOV device shall function correctly depends on the energy absorbed each time it functions. Furthermore, there is the inconvenience that they may short circuit in case of malfunction so that it is necessary to anticipate some other type of protection against this inconvenience. Furthermore these MOV devices have relatively high capacity. With regard to the GDT devices, it must be stated that these are slower acting devices that function by producing an electric arc in their interior when nominal voltage is surpassed. When in action impedance between their terminals diminishes drastically causing a short circuit. They have relatively small capacity making them ideal for use in telecommunications.
As a solution to these problems the use of GDT devices in combination with MOV devices connected in series between the terminals of the element to be protected is known. This combination has the advantage that taken together capacity is approximately equal to that of the GDT device (a few pF) and protection is similar to that of the MOV but without short-circuiting the line while the protection is in action. However it must be stated that the combination of these devices have one important inconvenience that is the GDT device needed to carry out effective protection must have a holdover voltage greater than the working voltage of the communication line, which requires the use of GDT devices with very high nominal voltage.
Due to the difference of capacity between the MOV device (high capacity) and the GDT device (very low capacity), the voltage capacitive divider formed by both devices causes almost all the voltage to be applied to the GDT device so that when a voltage surges occurs on the line, the GDT device turns the MOV device on, limiting the voltage absorbing the transient energy, but when the voltage surge ends and voltage on the line returns to its nominal value, if the holdover voltage of the GDT device is below maximum voltage on the line it beings to impulsively start and stop the capacitance of the MOV causing a malfunction and this is translated to noise in the channel something which is not admissible in telecommunications applications.